For many years balata and blends of balata with elastomeric or plastic materials were the principal materials used in the manufacture of top quality golf balls. Balata and balata-containing cover compositions posses the desirable property of ready adaption to molding and accordingly can be easily compression molded about a spherical wound core to produce a high quality golf ball.
The relative softness of the balata cover is advantageous in that an experienced golfer can apply a spin to balata-covered balls to control the ball in flight to produce a draw or a fade, or a backspin to cause the ball to "bite" or stop abruptly on contact with the green. Such playability properties are particularly important in short iron play and are exploited significantly by relatively skilled players.
However, balata covered golf balls are easily cut if mishit and golf balls produced with balata or balata-containing cover compositions have a relatively short life span.
Due to this negative property, balata and its synthetic substitutes, trans-polybutadiene and transpolyisoprene, have today been replaced essentially by new cover materials, primarily by a family of ionomers sold by E. I. duPont de Nemours & Company under the trademark SURLYN. These ionomers comprise copolymers of olefins, typically ethylene, with an alpha, beta ethylenically unsaturated carboxylic acid, such as methacrylic acid. Neutralization of a number of the acidic groups is effected with metal ions, such as zinc or sodium, resulting in a thermoplastic elastomer which has several advantages over balata when employed as the cover composition for golf balls.
In addition to cost-saving vis-a-vis balata, the hardness, resilience and other mechanical properties of these ionomers may be controlled in order to effect cut resistance, shear resistance, general durability and resilience. Such control can be exercised by selection of particular comonomers, molecular weight, degree of neutralization and the particular metal ions used.
The use of Surlyn as golf ball covers has been a major factor in the production of two-piece balls. These Surlyn balls for all practical purposes cannot be cut in play and they travel further when hit with a golf club as compared to the prior art balata covered balls. Regardless of these desirable properties, Surlyn covered golf balls have not proven desirable in short iron play, in that it is more difficult to impart spin to the Surly covered balls than to the balata covered counterpart. Further, frequently experienced golfers have noted that the Surlyn covered balls do not have a satisfactory "feel" or "click".
The golfing industry has therefore, for a number of years, attempted to develop a cover composition which possesses both the durability of the Surlyn formulations and the playability characteristics of the balata-based compositions.
In the mid-1980's, golf balls were introduced to the industry which were covered with a mixture of hard ionic resins and a thermoplastic polyurethane resin. The hard ionomers used in these blends had a hardness of about 94-95 on the Short C scale. While these balls have been generally satisfactory in providing the desirable qualities of playability and acceptable durability, these balls were disadvantageous in that they were short on distance when hit by an average golfer with an average swing velocity. Further, these prior art golf balls had a lower initial velocity when compared to prior art golf balls which were covered with a single or blend of hard ionomers. In the subject application, when a hard ionomer is referred to it is a Surlyn resin having a hardness of from about 94-96 on the Shore C scale.
Further, it has been noted that balls covered with the mixture of Surlyn and soft thermoplastic polyurethane tend to yellow and not to provide the bright white which can be obtained in traditional Surlyn covered balls. This deficiency results from the fact that the polyurethane resins tend to be yellow to off white in color.
Still further, the mixtures of Surlyns and polyurethanes are difficult to process and mold, such difficulties being inherent in all thermoplastic polyurethane resins. For example, it is difficult to obtain a homogeneous blend of Surlyns and polyurethanes in an extruder prior to injection molding the composition onto the preposition core.
It is felt that ionomers such as Surlyn ionomer are basically incompatible with polyurethane resins due to the backbone difference of each of these polymeric species. As a result of this incompatibility, delamination or separation in the manufacturing process may occur. Finally, such mixtures of Surlyns and polyurethane are expensive, due to the initial cost of the thermoplastic polyurethane and in compounding procedure requirements.
Attempts to modify the basic ionomer polyurethane compositions by such methods as adding additional hard ionomers to enhance toughness have been unsuccessful since the materials are basically incompatible at levels of greater than 15 to 20 percent ionomer. Further, blends of harder thermoplastic polyurethanes with hard ionomers have resulted in products lacking an adequate initial velocity.
It is, therefore, an object of the present invention to provide a golf ball which is cut resistant and provides the desirable playability features of balata-covered golf balls.
It is another object to provide a golf ball which is longer, i.e., does not have a distance shortcoming when struck with a golf club.
A further object of the present invention is to provide a golf ball which exhibits a bright white coloration and which does not yellow upon aging.
Still another object of this invention is to provide a golf ball cover composition which can be easily processed.
Lastly, it is an object of this invention to provide a golf ball cover composition which does not delaminate in either processing or on finished golf balls.